This invention relates to servo systems employing compound actuators which comprise a fine actuator portion and a coarse actuator portion, and, more particularly, to positioning the coarse actuator portion of the compound actuator.
In servo systems having compound actuators, such as for the transfer of data with data storage media, e.g., magnetic tape, the compound actuator provides both a large working dynamic range and high bandwidth. A typical compound actuator comprises a coarse actuator portion, such as a stepper motor, and a fine actuator portion,; such as a voice coil motor, mounted on the coarse actuator portion. Thus, in the example of data transfer, a data head can be translated between tracks over a full width of the magnetic tape using the coarse actuator portion, and can track follow the lateral movement of the track, using the fine actuator portion of the compound actuator.
The fine actuator portion of the compound actuator typically follows the track guiding disturbances, as determined by a servo sensor and detector which senses servo tracks, to position the data head in the center of the desired data track or tracks. It has relatively small mass and wide bandwidth and is thus able to follow high frequency disturbances. However, it has a very limited range of travel in order to provide the high bandwidth. The coarse actuator portion carries the fine actuator portion from track to track and also aids in centering the fine actuator portion.
In systems having one servo track per data track or set of parallel data tracks, the location of the actuator may have been determined by reading a track address recorded as a part of the servo track, for example, as illustrated by U.S. Pat. No. 15 5,121,270. However, it may be too costly to record addresses for the singly sensible servo tracks, and too time-consuming to read the address as compared to the short time required to access the data.
The data tracks are becoming increasingly smaller and closer together in order to increase data capacity, for example, of a given tape, by increasing the data track density. The increased data track density is handled by providing laterally indexed defined servo positions which are laterally equidistantly spaced across singly sensible servo tracks. As one example, 6 separate indexed defined servo positions are provided for a singly sensible servo track. It is not practical to record a separate address for each indexed defined servo position, and would prove highly costly to add precision sensors external to the servo sensors to attempt to determine the precise servo track which is being followed.
Ideally, a compound actuator positions the head at a desired index position, employing the coarse actuator portion, and then follows the index position of the defined servo track, employing the fine actuator portion. The coarse actuator portion is typically a stepper motor which is driven forward or backward by step drive pulses. The number of pulses are counted from a reference position, such as at the edge of the tape, and each index position is located at a step count of the stepper motor. Thus, the coarse actuator portion is moved to the step count of the desired indexed defined servo position.
The servo tracks are typically prerecorded and tend to be relatively fixed with respect to the tape, as does the indexed defined servo position. However, the tape tends to move laterally while-being guided across the tape head, and this movement tends to exceed the range of movement of the fine actuator portion. Hence, during the track following operation, both the coarse and fine actuator portions are actively controlled to achieve the track following requirements. The fine actuator portion has a high bandwidth response, and can follow fast changes in tape guiding. It also suppresses the effect of vibration and most shock disturbances to meet the track following requirements. However, its range of motion is limited, and it will typically not span the full dynamic range of motion required for following a data track over the full length of tape. The coarse actuator portion, which has a much slower response, but a very large stroke distance, is commanded to move to attempt to keep the fine actuator near it""s center of motion. The result is a continuously varying lateral position for the coarse actuator. Additionally, the coarse actuator moves the head over long distances from servo track to servo track, requiring a large number of steps.
Consequently, if steps are missed, for example, due to friction, backlash, non-linearity, or mis-stepping, the position control for the coarse actuator portion accumulates an error between the actual coarse position and the assumed coarse position. If the step count is lost or is no longer accurate due to missed steps., the step count may be reinitialized by moving the compound actuator to the reference position, such as at the edge of the tape. However, significant time is lost in reinitializing the compound actuator, reducing the performance of the tape drive.
An object of the present invention is to provide the correct coarse position of the coarse actuator portion of a compound actuator.
Disclosed are,a method and position control for a servo system for positioning a head laterally with respect to defined servo tracks. The!servo system comprises a servo sensor and detector for sensing lateral position of the head with respect to the defined servo tracks, and a compound actuator having a fine actuator portion for translating the head laterally with respect to the defined servo tracks, and a coarse actuator portion for translating the fine actuator portion laterally with respect to the defined servo tracks.
The position control is coupled to the servo sensor and detector and is coupled to the compound actuator for operating the compound actuator to position the actuator portions. The position control,provides position indications for the coarse actuator portion comprising predetermined nominal lateral position indications with respect to the defined servo tracks. The predetermined nominal lateral position indications may comprise a table relating the predetermined nominal lateral position indications to the defined servo tracks. The position control positions the coarse actuator portion at an estimated position with respect to the defined servo tracks, and responds to the servo sensor and detector to determine which servo track position is closest to the estimated position. The position control sets the coarse actuator portion position indication to the predetermined nominal lateral position indication of the determined defined servo track, thereby correcting the indication of the position of, the coarse actuator portion. This may comprise looking up the determined defined servo track in the table to determine the coarse actuator portion predetermined nominal lateral position indication of the closest defined servo track.